Determination of descending drive from single motor unit recordings: a model based approach

A number of descending drives tend to synchronise motor unit firings to particular frequencies during sustained voluntary contractions. Detection of these drives is usually performed using bivariate or multivariate measurements. We investigate the possibility of detecting these drives from single motor unit recordings. A computational model of the surface electromyogram was used to examine the variability of the instantaneous firing rate intervals of synchronised motor unit trains. The model generated distributions of this variability suggest that they arise from a mixture of two separate Gaussian distributions. A constrained version of the expectation-maximization algorithm is used to determine the component density means which are related to the descending drive frequencies. This algorithm is tested against a variety of drive frequencies and drive strengths.